OMO, back EMF brushed dc motor speed control

[SV650rules]

Originally Posted by madcockney

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

In the UK the electrical power suppliers are legally obliged to keep supply frequency within +/- 1% of 50 Hz.

Synchronous AC motors (single phase shaded pole) do produce a 'mains freq hum' (electrical noise) but are normally quiet mechanically.

Well done to Bibio though for his quest for a better, cheaper way.

[Bibio]

here is the realtime frequency of the UK grid. as the needle moves up and down this would be the effect of an AC motor speed drift. this would sound like a piano note not being constant or slightly out of tune.

http://www.dynamicdemand.co.uk/grid.htm

now for even more complicated issues surrounding the use of AC motors, most of the older TT's that use AC motors in the UK have a 110vac motor and as such have to use capacitors to bring the voltage down. thats all very well but you also have to remember that the UK mains voltage was taken down to 230vac then add to that the tolerance of cheep capacitors being +-20% then add in degradation of component and you might as well turn the TT with your hand. nowadays though there is another way to 'lock' AC voltage by converting it to DC then set the frequency using crystal oscillators then turn that DC back to AC, this solution works great but you still get AC motor magnetic radiation which can cause havoc with some MM cartridges.

my design uses a dual (+ 0 -) linier DC supply so even if the mains does shift the output DC stays the same.

[DarrenSV650S]

Originally Posted by Bibio

thats all very well but you also have to remember that the UK mains voltage was taken down to 230vac

Only on paper. It's still 240